Basement Window Size Calculator (Ontario Egress Check)
Enter your openable sash dimensions and select your unit. This tool checks your window against Ontario Building Code egress requirements.
Understanding the 0.38m² Egress Rule
Ontario Building Code Section 9.9.10.1 sets the life-safety standard for basement egress. The rule requires a minimum unobstructed openable area of 0.38 square meters, equal to 4.1 square feet. This is the true opening size when the sash is fully open, not the full frame size listed on a product label. During permit review and final inspection, officials assess the clear opening because occupants must exit in an emergency. If the opening is below 0.38m², the window fails code even if everything else in the suite appears compliant. Using an egress calculator early helps owners, contractors, and designers avoid redesign costs, failed inspections, and construction delays today.
The 380mm Minimum Dimension Catch
Many homeowners miss the second test: no single openable dimension can be less than 380mm, or about 15 inches. In practice, a window can meet the 0.38m² area threshold and still fail if either the clear width or clear height is under 380mm. This dimension rule exists so a person can pass through the opening without obstruction. Measure the clear open sash path, not rough openings, trim lines, or manufacturer nominal sizes. If one dimension is short, concrete cutting and a larger opening are required to achieve OBC-compliant emergency egress for basement bedrooms.
Standard basement slider windows almost always fail the Ontario Building Code egress test. In the event of a fire, a fully geared firefighter must be able to pull a tenant through the exact opening. If your existing windows don't meet the math, the foundation concrete must be cut.
The Ontario Building Code (OBC Sec. 9.9.10.1) dictates that an egress window must have a minimum clear opening area of 0.38 square meters (4.1 sq. ft.). Additionally, no single dimension (width or height) can be less than 380mm (15 inches).
A legal secondary suite operates as an isolated life-safety capsule. While a separate entrance with a 45-minute fire resistance rating is required for the unit itself, every individual bedroom within that basement must also have its own compliant egress window for emergency escape.